Java线程：新特征-锁

Java线程：新特征-锁
在Java5中，专门提供了锁对象，利用锁可以方便的实现资源的封锁，用来控制对竞争资源并发访问的控制，这些内容主要集中在java.util.concurrent.locks 包下面，里面有三个重要的接口Condition、Lock、ReadWriteLock。








Condition

Condition 将 Object 监视器方法（wait、notify 和 notifyAll）分解成截然不同的对象，以便通过将这些对象与任意 Lock 实现组合使用，为每个对象提供多个等待 set （wait-set）。



Lock

Lock 实现提供了比使用 synchronized 方法和语句可获得的更广泛的锁定操作。



ReadWriteLock

ReadWriteLock 维护了一对相关的锁定，一个用于只读操作，另一个用于写入操作。




有关锁的介绍,API文档解说很多，看得很烦，还是看个例子再看文档比较容易理解。






import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
* Java线程：锁
*
* @author leizhimin 2009-11-5 10:57:29
*/
public class Test {
        public static void main(String[] args) {
                //创建并发访问的账户
                MyCount myCount = new MyCount("95599200901215522", 10000);
                //创建一个锁对象
                Lock lock = new ReentrantLock();
                //创建一个线程池
                ExecutorService pool = Executors.newCachedThreadPool();
                //创建一些并发访问用户，一个信用卡，存的存，取的取，好热闹啊
                User u1 = new User("张三", myCount, -4000, lock);
                User u2 = new User("张三他爹", myCount, 6000, lock);
                User u3 = new User("张三他弟", myCount, -8000, lock);
                User u4 = new User("张三", myCount, 800, lock);
                //在线程池中执行各个用户的操作
                pool.execute(u1);
                pool.execute(u2);
                pool.execute(u3);
                pool.execute(u4);
                //关闭线程池
                pool.shutdown();
        }
}

/**
* 信用卡的用户
*/
class User implements Runnable {
        private String name;                //用户名
        private MyCount myCount;        //所要操作的账户
        private int iocash;                 //操作的金额，当然有正负之分了
        private Lock myLock;                //执行操作所需的锁对象

        User(String name, MyCount myCount, int iocash, Lock myLock) {
                this.name = name;
                this.myCount = myCount;
                this.iocash = iocash;
                this.myLock = myLock;
        }

        public void run() {
                //获取锁
                myLock.lock();
                //执行现金业务
                System.out.println(name + "正在操作" + myCount + "账户，金额为" + iocash + "，当前金额为" + myCount.getCash());
                myCount.setCash(myCount.getCash() + iocash);
                System.out.println(name + "操作" + myCount + "账户成功，金额为" + iocash + "，当前金额为" + myCount.getCash());
                //释放锁，否则别的线程没有机会执行了
                myLock.unlock();
        }
}

/**
* 信用卡账户，可随意透支
*/
class MyCount {
        private String oid;         //账号
        private int cash;             //账户余额

        MyCount(String oid, int cash) {
                this.oid = oid;
                this.cash = cash;
        }

        public String getOid() {
                return oid;
        }

        public void setOid(String oid) {
                this.oid = oid;
        }

        public int getCash() {
                return cash;
        }

        public void setCash(int cash) {
                this.cash = cash;
        }

        @Override
        public String toString() {
                return "MyCount{" +
                                "oid='" + oid + '\'' +
                                ", cash=" + cash +
                                '}';
        }
}





张三正在操作MyCount{oid='95599200901215522', cash=10000}账户，金额为-4000，当前金额为10000
张三操作MyCount{oid='95599200901215522', cash=6000}账户成功，金额为-4000，当前金额为6000
张三他爹正在操作MyCount{oid='95599200901215522', cash=6000}账户，金额为6000，当前金额为6000
张三他爹操作MyCount{oid='95599200901215522', cash=12000}账户成功，金额为6000，当前金额为12000
张三他弟正在操作MyCount{oid='95599200901215522', cash=12000}账户，金额为-8000，当前金额为12000
张三他弟操作MyCount{oid='95599200901215522', cash=4000}账户成功，金额为-8000，当前金额为4000
张三正在操作MyCount{oid='95599200901215522', cash=4000}账户，金额为800，当前金额为4000
张三操作MyCount{oid='95599200901215522', cash=4800}账户成功，金额为800，当前金额为4800

Process finished with exit code 0



从上面的输出可以看到，利用锁对象太方便了，比直接在某个不知情的对象上用锁清晰多了。



但一定要注意的是，在获取了锁对象后，用完后应该尽快释放锁，以便别的等待该锁的线程有机会去执行。
在上文中提到了Lock接口以及对象，使用它，很优雅的控制了竞争资源的安全访问，但是这种锁不区分读写，称这种锁为普通锁。为了提高性能，Java提供了读写锁，在读的地方使用读锁，在写的地方使用写锁，灵活控制，在一定程度上提高了程序的执行效率。



Java中读写锁有个接口java.util.concurrent.locks.ReadWriteLock，也有具体的实现ReentrantReadWriteLock，详细的API可以查看JavaAPI文档。



下面这个例子是在文例子的基础上，将普通锁改为读写锁，并添加账户余额查询的功能，代码如下：





import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
* Java线程：锁
*
* @author leizhimin 2009-11-5 10:57:29
*/
public class Test {
        public static void main(String[] args) {
                //创建并发访问的账户
                MyCount myCount = new MyCount("95599200901215522", 10000);
                //创建一个锁对象
                ReadWriteLock lock = new ReentrantReadWriteLock(false);
                //创建一个线程池
                ExecutorService pool = Executors.newFixedThreadPool(2);
                //创建一些并发访问用户，一个信用卡，存的存，取的取，好热闹啊
                User u1 = new User("张三", myCount, -4000, lock, false);
                User u2 = new User("张三他爹", myCount, 6000, lock, false);
                User u3 = new User("张三他弟", myCount, -8000, lock, false);
                User u4 = new User("张三", myCount, 800, lock, false);
                User u5 = new User("张三他爹", myCount, 0, lock, true);
                //在线程池中执行各个用户的操作
                pool.execute(u1);
                pool.execute(u2);
                pool.execute(u3);
                pool.execute(u4);
                pool.execute(u5);
                //关闭线程池
                pool.shutdown();
        }
}

/**
* 信用卡的用户
*/
class User implements Runnable {
        private String name;                //用户名
        private MyCount myCount;        //所要操作的账户
        private int iocash;                 //操作的金额，当然有正负之分了
        private ReadWriteLock myLock;                //执行操作所需的锁对象
        private boolean ischeck;        //是否查询

        User(String name, MyCount myCount, int iocash, ReadWriteLock myLock, boolean ischeck) {
                this.name = name;
                this.myCount = myCount;
                this.iocash = iocash;
                this.myLock = myLock;
                this.ischeck = ischeck;
        }

        public void run() {
                if (ischeck) {
                        //获取读锁
                        myLock.readLock().lock();
                        System.out.println("读：" + name + "正在查询" + myCount + "账户，当前金额为" + myCount.getCash());
                        //释放读锁
                        myLock.readLock().unlock();
                } else {
                        //获取写锁
                        myLock.writeLock().lock();
                        //执行现金业务
                        System.out.println("写：" + name + "正在操作" + myCount + "账户，金额为" + iocash + "，当前金额为" + myCount.getCash());
                        myCount.setCash(myCount.getCash() + iocash);
                        System.out.println("写：" + name + "操作" + myCount + "账户成功，金额为" + iocash + "，当前金额为" + myCount.getCash());
                        //释放写锁
                        myLock.writeLock().unlock();
                }
        }
}

/**
* 信用卡账户，可随意透支
*/
class MyCount {
        private String oid;         //账号
        private int cash;             //账户余额

        MyCount(String oid, int cash) {
                this.oid = oid;
                this.cash = cash;
        }

        public String getOid() {
                return oid;
        }

        public void setOid(String oid) {
                this.oid = oid;
        }

        public int getCash() {
                return cash;
        }

        public void setCash(int cash) {
                this.cash = cash;
        }

        @Override
        public String toString() {
                return "MyCount{" +
                                "oid='" + oid + '\'' +
                                ", cash=" + cash +
                                '}';
        }
}





写：张三正在操作MyCount{oid='95599200901215522', cash=10000}账户，金额为-4000，当前金额为10000
写：张三操作MyCount{oid='95599200901215522', cash=6000}账户成功，金额为-4000，当前金额为6000
写：张三他弟正在操作MyCount{oid='95599200901215522', cash=6000}账户，金额为-8000，当前金额为6000
写：张三他弟操作MyCount{oid='95599200901215522', cash=-2000}账户成功，金额为-8000，当前金额为-2000
写：张三正在操作MyCount{oid='95599200901215522', cash=-2000}账户，金额为800，当前金额为-2000
写：张三操作MyCount{oid='95599200901215522', cash=-1200}账户成功，金额为800，当前金额为-1200
读：张三他爹正在查询MyCount{oid='95599200901215522', cash=-1200}账户，当前金额为-1200
写：张三他爹正在操作MyCount{oid='95599200901215522', cash=-1200}账户，金额为6000，当前金额为-1200
写：张三他爹操作MyCount{oid='95599200901215522', cash=4800}账户成功，金额为6000，当前金额为4800

Process finished with exit code 0



在实际开发中，最好在能用读写锁的情况下使用读写锁，而不要用普通锁，以求更好的性能。